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1.
喜马拉雅造山带的部分熔融与淡色花岗岩成因机制 总被引：1，自引：0，他引：1
喜马拉雅造山带核部由高级变质岩和淡色花岗岩组成,是研究大陆碰撞造山带部分熔融与花岗岩成因的天然实验室.基于最新研究成果,探讨了喜马拉雅造山带核部变质作用的条件、类型以及P-T轨迹、部分熔融的方式与程度及熔体成分以及变质作用与部分熔融的时间和持续过程.相关证据表明,造山带核部经历了高压麻粒岩相至榴辉岩相变质作用,具有以增温增压进变质和近等温降压退变质为特征的顺时针型P-T轨迹.这些高压变质岩石发生了长期持续的高温变质与部分熔融.在泥质岩石的进变质过程中白云母和黑云母脱水熔融可以形成不同成分的熔体.同时,总结了淡色花岗岩的形成时间、地球化学特征和源区熔融方式,结果表明碰撞造山过程中加厚下地壳的脱水熔融形成了喜马拉雅造山带的淡色花岗岩. 相似文献
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下地壳岩石的深熔作用不仅与麻粒岩和花岗岩的形成具有紧密关联,而且在某些构造环境下会对地球动力学演化过程造成影响,因此,对深熔岩石的变质P-T条件的准确估算及对其熔融机制的限定具有重要意义。基于内洽性热力学数据库、THERMOCALC/Perple＿X等程序以及适当的固溶体活度模型,变质相平衡模拟已逐渐成为变质岩石学的常规研究方法,广泛应用于推断天然岩石的矿物共生序列、估算岩石的变质P-T条件等。不仅如此,如果有适当的熔体活度模型,变质相平衡模拟还能估算深熔岩石的熔融温度、压力及熔体比例,以及限定其涉及的熔融反应,并计算熔体、转熔矿物及残余矿物的成分等。针对不同成分岩石,包括花岗质岩石、变泥质岩、变基性岩及橄榄岩在不同压力下产生的熔体,前人陆续提出了对应的活度模型,并且其有效性得到了相关实验数据的验证。随着近年来熔体活度模型的不断更新和完善,变质相平衡模拟有望成为研究自然界深熔岩石的常规方法,为相关的麻粒岩和花岗岩成因研究、相关地球动力学演化研究提供新的思路。 相似文献
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麻粒岩相变质作用与花岗岩成因-Ⅱ:变质泥质岩高温-超高温变质相平衡与S型花岗岩成因的定量模拟 总被引：12，自引：12，他引：0
高温-超高温变质岩石的矿物组合及组构特点取决于不同的进变熔融反应,不同程度的熔体丢失以及不同程度的退变反应三种过程的综合效应。利用相平衡定量研究方法可以很好地模拟进变熔融反应的类型、P-T条件、熔体含量及其丢失行为、以及熔融过程中熔体与残余物的化学成分变化等,这对探讨高温-超高温变质作用过程以及花岗岩的成因非常重要。对平均泥质岩(APR)进行相平衡模拟表明变质泥质岩在等压(0.8GPa)升温熔融过程中可发生5种熔融反应:饱和流体固相线、白云母脱水熔融、黑云母熔融、钾长石-石榴石熔融和铝铁镁矿物熔融,后两种熔融反应主要发生在超高温条件下。减压过程中发生怎样的熔融反应受减压温度控制:在麻粒岩相(如850℃)减压可发生钾长石熔融、黑云母熔融和钾长石-石榴石熔融反应;在高角闪岩相(如750℃)减压主要发生白云母脱水熔融和钾长石熔融;在超高温麻粒岩相(如950~1000℃)减压主要发生钾长石-石榴石熔融和铝铁镁矿物熔融。熔体成分受熔融反应和P-T条件控制,如在高角闪岩相发生的饱和流体固相线和白云母脱水熔融可形成弱过铝的奥长花岗质和二长花岗质熔体;在麻粒岩相发生的黑云母熔融和钾长石熔融形成的熔体具有强过铝的二长花岗岩成分;在中压超高温发生的钾长石-石榴石熔融和铝铁镁矿物熔融形成强过铝的二长(钾长)花岗岩质熔体,可形成石榴石花岗岩;在低压超高温下发生的铝铁镁矿物熔融可形成堇青石花岗岩。除了极端超高温下的铝铁镁矿物熔融外,其它熔融反应都会使残余物的成分更贫硅,贫Na_2O和K_2O,富FeO和MgO,但Al_2O_3和Mg#基本不变。高温-超高温下发生深熔的岩石只记录降温过程形成的固相线组合,但固相线的类型与温度条件取决于熔体的丢失行为。在不丢失熔体或者获得熔体的岩石中,岩石最后只记录流体饱和固相线组合;发生熔体部分丢失的岩石会记录缺流体固相线组合,并且熔体丢失越多,缺流体固相线的温度越高;发生全部流体丢失的岩石可记录岩石所达到的最高温度。因此,在一个麻粒岩相区,甚至一个野外露头上不同部位的岩石记录不同的P-T条件。熔体丢失是导致使麻粒岩相组合在升温过程中发生超高温变质,在降温过程中得以部分保存的重要条件。发生部分熔融的高级变质岩中随着温度升高,熔体含量增加,会发生锆石分解,只有在降温过程中发生锆石结晶,因此,麻粒岩中新生锆石只记录降温过程到固相线及以后的年龄,一般不会记录麻粒岩相峰期时代。对泥质高压麻粒岩来说,如果经历ITD型变质演化,会发生递进减压熔融,变质反应易于达到平衡,但如果减压速度快并使岩石直接抬升到地壳浅部,会出现一些ITD型结构标志,如残留金红石、蓝晶石,或在石榴石周围出现堇青石的反应冠状体等,此时锆石记录的退变质年龄会与峰期变质年龄相差不大(如10~30Myr);但如果泥质高压麻粒岩减压至中、深地壳,受其中有滞留熔体影响易于发育IBC型结构特征,表现为麻粒岩组合被(中压)角闪岩相组合叠加,在泥质岩中出现黑云母+夕线石构成的暗色条带,或者出现退变白云母和含白云母的浅色体。在中、深地壳经历IBC过程的麻粒岩锆石记录的退变质年龄会与峰期年龄相差很大(如~100Myr)。高级变质岩中由于出现熔体使水流体活度降低,麻粒岩作为排除部分熔体的残余物,其水活度更低。从这一角度来说,水活度低是麻粒岩相变质作用的结果,而不是条件。某些麻粒岩区之所以出现多期麻粒岩相变质叠加受流体行为控制。在亚固相线下流体饱和岩石变质熔融作用从饱和水固相线开始,然后依次发生含水矿物的脱水熔融和无水矿物熔融,这一过程中流体是内部缓冲的,在麻粒岩相温度峰期形成一组平衡矿物组合,难以保留峰期之前的信息。而流体不饱和岩石(如已形成的麻粒岩或岩浆侵入体)变质作用受外部注入流体控制,与构造变形密切相关。如果发生两期麻粒岩相变质叠加变质,在强应变域会形成晚期麻粒岩组合;在弱应变域,会出现两期麻粒岩组合,其中晚期矿物表现为反应冠状体或细粒交生体;而在一些应变非常弱的区域,可能只保留早期矿物组合。 相似文献
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新疆阿尔泰高级变质带中淡色花岗质脉体的特征及成因模拟 总被引：1，自引：0，他引：1
在新疆阿尔泰地区的高级变质带中广泛发育着一系列规模不等的透镜状和条带状的浅色脉体,主要有含Al2SiO5的淡色花岗岩脉、白云母斜长花岗岩脉和白云母二长花岗岩脉。主量元素分析表明这3类脉体的A/CNK=1.1~2.14,属于S型花岗岩。从白云母二长花岗岩脉→白云母斜长花岗岩脉→含Al2SiO5的淡色花岗岩脉,SiO2的含量增高,Al2O3、Na2O、K2O的含量降低。在NKFMASH体系中的pT视剖面图上进行相平衡分析和熔体成分计算表明,含Al2SiO5的浅色花岗岩脉和白云母斜长花岗岩脉的熔体形成与蓝晶石型变质带的抬升降压过程有关,其熔融温度没有超过白云母脱水熔融反应,并且含Al2SiO5的淡色花岗岩脉不是由生成的熔体直接结晶形成的,而需要经历一定的碱性组分随流体迁移丢失;白云母二长花岗岩脉的熔体形成主要以增温为主,并与白云母脱水熔融反应有关。十字石的脱水熔融反应对熔体形成有明显贡献。 相似文献
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藏南错那淡色花岗岩LA-MC-ICP-MS锆石U-Pb年龄、岩石地球化学及其地质意义 总被引：10，自引：1，他引：9
藏南错那淡色花岗岩位于喜马拉雅造山带的东部。对其进行LA-MC-ICP-MS锆石U-Pb定年,结果显示,结晶年龄为17.7±0.3Ma,代表中新世的地壳深熔作用。淡色花岗岩样品具有高的Si O2(74.46%~75.57%)、Al2O3(14.07%~14.64%)和K2O(4.19%~4.85%)含量,高的K2O/Na2O值(1.09~1.31)和A/CNK值(1.15~1.25),富集Rb、Th和U,亏损Ba、Nb、Sr、Zr等元素,显示高的Rb/Sr值(17.75~29.50)和强烈的负Eu异常(δEu=0.18~0.26),属于壳源成因的高钾钙碱性过铝质S型花岗岩。样品具有高的Isr值(0.78982~0.79276)和低的εNd(t)值(-19.5~-18.2),可与大喜马拉雅结晶杂岩(GHC)中的变泥质岩对比,暗示其来自变泥质岩的部分熔融。样品的Isr值较高,而Sr浓度较低,且随着Ba浓度的增加,Rb/Sr值逐渐降低,表明淡色花岗岩是无水条件下白云母部分熔融的产物,部分熔融可能与藏南拆离系(STDS)伸展拆离导致的构造减压有关。错那淡色花岗岩的形成反映了地壳伸展减薄背景下,构造减压导致的中下地壳中含水矿物脱水熔融,并沿STDS上升侵位的动力学过程。 相似文献
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《岩石学报》2016,(6)
麻粒岩相岩石作为洞察下地壳的窗口一直备受重视。二十世纪九十年代以来麻粒岩研究的一个重要进展是利用变质相平衡的定量研究方法模拟岩石中所发生的深熔变质反应、熔体成分变化、及熔体丢失对变质矿物组合的影响等。本文利用KASH、NKASH和KFMASH等简单体系的相平衡关系,做出P-T投影图、组分共生图解和基于固定全岩成分的P-T视剖面图解,并结合有关实验岩石学结果,讨论了高温和超高温条件下变质泥质岩和杂砂岩中的变质熔融反应、矿物组合、全岩成分与P-T条件之间的相互关系。多数变质泥质岩和杂砂岩中饱和流体固相线熔融反应可利用NKASH体系中有水流体参与的熔融反应模拟,在没有外来流体注入时,这些反应可形成3mol%熔体。在不同体系中白云母脱水熔融反应型式及其P-T条件不同,如在NKASH和KFMASH体系中模拟计算的白云母脱水熔融反应与相应的实验结果相似,分别控制了白云母分解熔融的温度下限和上限;白云母的分解温度会随着其中Fe、Mg和Ti含量的增加而升高,也随着共生斜长石中钙长石组分增加而升高,泥质岩中白云母脱水熔融可以形成~10mol%熔体。在KFMASH体系中黑云母脱水熔融反应表现为4条单变反应,其理论计算的温度比实验模拟的结果低一些。在NCKFMASH体系或实际岩石中黑云母脱水熔融反应为滑动反应,如NCKFMASH体系中黑云母从其开始熔融到最后消失在泥质岩中可跨越~100℃,在杂砂岩中可跨越30~50℃。黑云母的稳定温度随着镁值升高而升高,其稳定上限受钛影响更大,黑云母脱水熔融可以形成超过30mol%~40mol%熔体。KFMASH体系中的相平衡模拟表明以出现斜方辉石+夕线石和假蓝宝石为特征的超高温组合易于出现于富镁泥质岩中,而对正常成分泥质岩在达到1000℃的超高温条件下,主要出现石榴石+夕线石(即夕线榴),该组合在更高温度反应形成假蓝宝石+尖晶石。利用饱和水固相线反应和白云母与黑云母分解反应可以更好地限定不同的变质相。如中压和低压条件下低角闪岩相和高角闪岩相的界限可利用NKASH体系中有水流体和白云母参与的熔融反应和亚固相线条件下的白云母分解反应限定;实验确定的泥质岩中黑云母开始熔融与消失的反应可分别用于限定高角闪岩相与(正常)麻粒岩相的界限,以及(正常)麻粒岩相和超高温麻粒岩相的界限。因此,从矿物组合角度,正常麻粒岩相可限定在黑云母开始熔融到完全消失的温度范围,超高温麻粒岩相可限定在黑云母消失(有石英存在)之后的温度范围。 相似文献
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印度大陆俯冲过程中的高压变质与深熔作用：东喜马拉雅构造结南迦巴瓦杂岩的相平衡模拟研究 总被引：5，自引：5，他引：0
位于东喜马拉雅构造结的南迦巴瓦杂岩是高喜马拉雅结晶岩系的一部分,是印度大陆深俯冲到欧亚板块之下经历了高压变质作用的产物。基于岩相学和矿物化学研究,本文对南迦巴瓦杂岩中的泥质变质岩进行了相平衡模拟研究。结果表明,泥质岩石经历了高压麻粒岩相变质作用,峰期矿物组成是石榴石+蓝晶石+黑云母+斜长石+钾长石+石英+金红石,峰期变质条件是~820℃,13.0~13.5kb,表明印度大陆至少俯冲到了约45km深度,构成了青藏高原的加厚下地壳。高压泥质变质岩在进变质和峰期变质过程中经历了白云母和黑云母脱水反应引起的部分深熔,熔融程度可达27vol%,形成了花岗质成分的熔体,构成了喜马拉雅造山带淡色花岗岩的源区。因此,青藏高原具有一个深熔融的中下地壳,为其侧向流动提供了有利的流变学环境。 相似文献
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南迦巴瓦地区广泛出露的中下地壳变基性岩部分熔融形成的层状混合岩和淡色花岗岩,为研究部分熔融过程中榍石的地球化学行为对熔体的微量元素组成的影响提供了良好的机会。相对于源岩或熔融残留体,淡色体亏损Ti、V、REE、Y、Nb、Ta、U等元素,与混合岩中榍石的微量元素特征互补。混合岩、淡色体和榍石微量元素特征表明南迦巴瓦角闪岩部分熔融形成的淡色体的微量元素特征主要受控于榍石的地球化学行为。角闪岩脱水部分熔融过程中,由于长英质熔体的低Ti溶解度,榍石以未熔残留体形式存在于暗色体中,导致熔体亏损Ti、REE、Nb、Ta、V、U等元素和Sr/Y比值相对升高。关键元素在榍石和熔体之间的配分系数受熔体成分影响明显。角闪岩中变质榍石DNb/Ta<1,因此变质榍石残留导致熔体Nb/Ta相对于源岩升高;而高Si-Al花岗质熔体中榍石DNb/Ta>1,因此与高Si-Al熔体平衡的榍石的分离（转熔或结晶分异）将导致熔体Nb/Ta比值相对源岩降低。榍石在部分熔融过程中的微量元素效应为理解变基性岩部分熔融产生熔体的地球化学特征提供新的认识。 相似文献
10.
藏南定结地区早中新世淡色花岗岩的形成机制及其构造动力学意义 总被引：13，自引：6，他引：7
定结日玛那穹窿位于高喜马拉雅带中段,由花岗片麻岩、变泥质岩、变基性岩及大量淡色花岗岩等组成,经历了角闪岩-麻粒岩相变质作用。为厘定淡色花岗岩的形成机制以及与高级变质岩的关系,我们对淡色花岗岩和高级变质岩进行了全岩元素和Sr和Nd同位素组成和SHRIMP锆石U-Pb地质年代学测试。全岩元素和Sr-Nd同位素测试结果揭示淡色花岗岩具有以下特征:(1)高SiO2 (>72%),高Al2O3 (>12%)和高A/CNK比值 (>1.0);(2)高Rb,低Sr,高Rb/Sr比值(>1.0);(3)高∑REE和明显的负Eu异常;(4)高Sr同位素初始比值(0.7621~0.8846)和低εNd(t)值(-13.0~-20.2)。淡色花岗岩的高Rb/Sr比值和Sr-Nd同位素系统特征表明其形成机制为主要为白云母脱水部分熔融作用,源区为由花岗片麻岩和变泥质岩组成的混合源区。SHRIMP锆石U-Pb年代学研究揭示出定结地区淡色花岗岩具有21.0±0.7Ma和15.8±0.1Ma 2期年龄,花岗片麻岩的锆石变质增生边年龄为22.2±1.4Ma,与该区的榴辉岩退变质年龄一致。这些数据共同表明,花岗片麻岩和 变泥质岩在22~21Ma发生高级变质和深熔作用,形成早期淡色花岗岩岩浆,在~16Ma进一步深熔,形成晚期淡色花岗岩岩浆。 相似文献
11.
The Higher Himalayan Crystalline Sequence (HHCS) provides an excellent natural laboratory to study continental subduction, crustal melting and tectonic evolution of orogenic belt generated through the collision of India with Eurasia. Our petrological study and phase equilibrium modeling reveal that the pelitic migmatites in the HHCS of Yadong region, east-central Himalaya, preserve an early mineral assemblage garnet, kyanite, biotite, quartz, plagioclase, K-feldspar, rutile and ilmenite, and a late sillimanite- and/or cordierite-bearing assemblage, and underwent the high pressure (HP) and high temperature (HT) granulite-facies metamorphism and associated partial melting under P–T conditions of ca. 12 kbar and 825–845 °C, followed by nearly isothermal decompression and isobaric cooling. The anatexis of the migmatites occurred dominantly through dehydration-melting of both muscovite and biotite during the prograde metamorphism. The melt produced in the peak metamorphic conditions is about 20 to 30 vol.% of the rocks, and a significant amount of melt has been extracted from the source leading to the formation of Himalayan leucogranites. The zircon U–Pb dating data shows that the migmatites probably witnessed a prolonged melting episode that began at ca. 30 Ma and lasted to ca. 20 Ma. These results show that the thickening lower crust of the Himalayan orogen experienced long-lived and continued HP and HT metamorphism and pervasive anatexis, supporting the models on channel flow. 相似文献
12.
The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Aktyuz metamorphic terrain, in the southern of Kokchetav‐North Tianshan belt, involving progressive metamorphic recrystallization of mafic rock to eclogite and associated behavior of zircon. Zircons in eclogites are mainly fine grains (5 to 20 μm), and preferentially concentrated with rutile/ilmenite. They also occur as individual grains or clusters in amphibole coronas of garnet. A few larger grains commonly preserve inherited cores and evidence of dissolution and metamorphic outgrowths. Zircon grains separated from amphibolites show inherited zircons with typically magmatic feature, although this become progressively blurred in response to resorption and recrystallization. Mineral inclusions represent epidote‐amphibolite facies in the prograde metamorphism, and the embayed boundary between recrystallized domains and inherited zircons suggest fluid/melt participation. The metamorphic domains are mainly simple overgrowth around the inherited cores or recrystallization domains. The absence of peak metamorphic mineral inclusions and steep pattern of MREE‐HREE indicate no sufficient garnet formed before the metamorphic zircon overgrowth. A tiny rim with homogeneously bright CL image can be distinguished in most zircons. Amphibole inclusions have similar compositions to those in the coronas of garnets, suggesting a retrograde metamorphic origin. The inherited zircon crystallized at 880‐730 Ma, revealing similar age range to the gneiss in Aktyuz area, whereas metamorphic zircon dates prograde metamorphism at 497.9 ±1.4 Ma. In this case, the bulk Zr budget in rocks will become locked into Zr‐bearing minerals during the mafic magma intrusion, when the inherited zircon melting and resorption. The texture shows that metamorphic zircon grew both in the prograde and retrograde stage, and Zr‐bearing magmatic minerals and rutile/ilmenite are by far the main source of Zr for the two stages, respectively. 相似文献
13.
Palaeoproterozoic metasedimentary migmatite reflects the highest temperature parts of a regional aureole at Mt Stafford, central Australia, comprising rocks that experienced 500–800 °C at ≈3 kbar. Whole‐rock major element concentrations are correlated with Zr content, psammitic compositions having nearly twice the Zr content of pelitic compositions. Zirconium is concentrated in mesosome compared with leucosome. Zircon is largely detrital, mostly lacking any overgrowth contemporary with migmatite formation. Comparatively small proportions of micro‐zircon (<10 μm) in sub‐solidus rocks are mostly hosted by quartz and plagioclase. Much higher proportions (three to five times) of micro‐zircon in migmatite are hosted by prograde K‐feldspar, cordierite and biotite. T–X and P–T NCKFMASHTZr pseudosections constructed using thermocalc model the distribution of Zr between solid and silicate liquid phases. Half of the detrital zircon (~100 ppm Zr) is predicted to be dissolved into silicate liquid at ≈800 °C and all dissolved by 850 °C, if all zircon is involved in the equilibration volume. Melt segregation at relatively low temperature is predicted to enrich the residuum in Zr, consistent with the observed distribution of Zr between mesosome and leucosome. The limited development of metamorphic zircon rims or overgrowths at Mt Stafford is explained by three concurrent processes: (i) Zr liberated during prograde metamorphism formed micro‐zircon, rather than following the prediction that Zr will partition into silicate liquid; (ii) some detrital zircon was probably armoured by other rock‐forming minerals, reducing Zr content in the effective bulk rock composition; and (iii) small proportions of melt loss during migmatization removed Zr that otherwise would have been available to form metamorphic rims. 相似文献
14.
麻粒岩的研究进展与方法 总被引：2，自引：0，他引：2
近年来,有关麻粒岩的研究取得了长足进展,本文讨论了4个相关问题:(1)麻粒岩的大地构造环境与P-T轨迹。麻粒岩可以形成于4种大地构造环境中:(a)碰撞造山带以形成高压麻粒岩为特征,为中压相系,包括曾位于地壳浅部的岩石经历构造埋深达到变质峰期后再折返的过程,为顺时针型P-T轨迹;也包括曾经历洋壳或陆壳俯冲形成的高压-超高压榴辉岩相岩石折返叠加变质形成的麻粒岩,P-T轨迹以减压为主。(b)地壳伸展区以形成低压麻粒岩为特征,并可达到超高温条件,其P-T轨迹为减压加热至温度峰期,随后发生等压或降压冷却。(c)岛弧或陆缘岩浆增生区的下地壳多为高压麻粒岩相,其中侵入的辉长岩首先经历等压冷却,然后再经历升温升压进变质过程。(d)太古宙克拉通麻粒岩相表壳岩呈皮筏状分布于TTG片麻岩内部,多达到超高温条件,发育逆时针型P-T轨迹,受太古宙特殊的垂直构造体制控制。(2)麻粒岩的进变质过程与流体行为。按照流体行为,麻粒岩的进变质过程分为3种型式:(a)流体饱和进变质过程,指岩石在饱水固相线之前达到流体饱和,随后发生饱水固相线熔融与含水矿物的脱水熔融,以及阶段性熔体丢失,导致岩石中水含量降低,缺流体固相线温度升高;在峰期之后的降温过程中,发生熔融反应的逆反应,或结晶反应,形成含水矿物,结晶反应终止于缺流体固相线。(b)流体不饱和或缺流体进变质过程,指岩石在进变质过程中会处于流体缺失状态,不会发生变质反应,岩石中原来的矿物组合以亚稳定状态保留至缺流体固相线后,才开始变质演化,因此经常形成一些不平衡结构。(c)流体过饱和进变质过程,指有过量水参与的熔融反应过程,也称为水化熔融,与熔体注入或局部汇聚有关;水化熔融过程中会更多地消耗斜长石、石英及辉石等无水矿物,导致残余物中富集角闪石和黑云母等含水矿物。(3)确定麻粒岩P-T条件的视剖面图方法。利用视剖面图方法分析麻粒岩的变质条件时,首先需要通过岩相学观察区分出峰期组合和最终组合;然后通过计算T-M(H2O)图解确定最终组合的含水量;最后利用所确定的水含量计算P-T视剖面图。利用P-T视剖面图分析麻粒岩的峰期变质条件时,首先找到峰期矿物组合在视剖面图上的稳定域,然后再结合有价值的矿物成分等值线确定P-T条件。特别需要注意的是,岩相学观察确定的峰期组合和最终组合都可能受局部结构域控制,与滞留熔体的不均匀分布或原地分凝有关,此时不能简单地用全岩成分模拟其相平衡关系。(4)相平衡模拟时需要选择有效的全岩成分。当选择实测全岩成分进行相平衡模拟时,首先需要检验其有效性,即检验实测全岩成分是否能够代表薄片中所观察到的相平衡关系。方法是计算有效全岩成分,并与实测全岩成分进行对比。对于成分不均匀的变质岩石,需要处理局部结构域的成分。分如下3种情况:(a)宏观尺度的结构域,可以分别取样;(b)微观尺度的结构域,需要在显微薄片中进行图像分析,针对不同结构域分别进行相平衡模拟;(c)由叠加或退变质形成的结构域,需要确定相应的变质反应,通过对反应配平,确定有效全岩成分。此外,文中还介绍了计算岩石中的水含量、O含量和各种矿物相含量的方法与注意事项。 相似文献
15.
Monazite behaviour during isothermal decompression in pelitic granulites: a case study from Dinggye,Tibetan Himalaya 总被引：1，自引：0，他引：1
Monazite is a key accessory mineral for metamorphic geochronology, but interpretation of its complex chemical and age zoning acquired during high-temperature metamorphism and anatexis remains a challenge. We investigate the petrology, pressure–temperature and timing of metamorphism in pelitic and psammitic granulites that contain monazite from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent isothermal decompression from pressure of >10 kbar to ~5 kbar at temperatures of 750–830 °C, and recorded three metamorphic stages at kyanite (M1), sillimanite (M2) and cordierite-spinel grade (M3). Monazite and zircon crystals were dated by microbeam techniques either as grain separates or in thin sections. U–Th–Pb ages are linked to specific conditions of mineral growth on the basis of zoning patterns, trace element signatures, index mineral inclusions (melt inclusions, sillimanite and K-feldspar) in dated domains and textural relationships with co-existing minerals. The results show that inherited domains (500–400 Ma) are preserved in monazite even at granulite-facies conditions. Few monazites or zircon yield ages related to the M1-stage (~30–29 Ma), possibly corresponding to prograde melting by muscovite dehydration. During the early stage of isothermal decompression, inherited or prograde monazites in most samples were dissolved in the melt produced by biotite dehydration-melting. Most monazite grains crystallized from melt toward the end of decompression (M3-stage, 21–19 Ma) and are chemically related to garnet breakdown reactions. Another peak of monazite growth occurred at final melt crystallization (~15 Ma), and these monazite grains are unzoned and are homogeneous in composition. In a regional context, our pressure–temperature–time data constrains peak high-pressure metamorphism within the GHC to ~30–29 Ma in Dinggye Himalaya. Our results are in line with a melt-assisted exhumation of the GHC rocks. 相似文献
16.
Magnesium isotopic compositions, along with new Sr–Nd–Pb isotopic data and elemental analyses, are reported for 12 Miocene tourmaline-bearing leucogranites, 15 Eocene two-mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic-magmatic-metamorphic evolution of the Himalayan orogeny. The gneisses, granulites and amphibolites represent samples of the Indian lower crust and display large range in δ26Mg from −0.44‰ to −0.09‰ in mafic granulites, −0.44‰ to −0.10‰ in amphibolites, and −0.70‰ to −0.03‰ in granitic gneisses. The average Mg isotopic compositions of the granitic gneisses (−0.19 ± 0.34‰), mafic granulites (−0.22 ± 0.17‰) and amphibolites (−0.25 ± 0.24‰) are similar, indicating the limited Mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites. The Eocene two-mica granites and Miocene leucogranites are characterized by large variations in elemental and Sr–Nd–Pb isotopic compositions. The leucogranites and two-mica granites have their corresponding (87Sr/86Sr)i varying from 0.7282 to 0.7860 and 0.7163 to 0.7191, (143Nd/144Nd)i from 0.511888 to 0.512040 and 0.511953 to 0.512076, 207Pb/204Pb from 15.7215 to 15.7891 and 15.7031 to 15.7317, 208Pb/204Pb from 38.8521 to 39.5286 and 39.2710 to 39.4035, and 206Pb/204Pb from 18.4748 to 19.0139 and 18.7834 to 18.9339. However, they have similar Mg isotopic compositions (−0.21‰ to +0.06‰ versus −0.24‰ to +0.09‰), which did not originate from fractional crystallization nor source heterogeneity. Based on hornblende/biotite/muscovite dehydration melting reaction and Mg isotopic variations in two-mica granites and leucogranites with the proceeding metamorphism, along with elemental discrimination diagrams, Eocene two-mica granites and Miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting, respectively. Mg isotopic compositions of Eocene two-mica granites become heavier compared to the source because of residues of isotopically light garnet in the source; while those of Miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region. Thus, a new model for crustal anatexis and Himalayan orogenesis was proposed based on the Mg isotope fractionation in the leucogranites and metamorphic rocks. This model emphasizes a successive process from Indian continental subduction to rapid exhumation of the Higher Himalayan Crystalline Series (HHCS). The former underwent high-temperature (HT) and high-pressure (HP) granulite-facies prograde metamorphism, which resulted in the hornblende dehydration melting and the formation of Eocene two-mica granites; while the latter experienced amphibolite-facies retrogression and decompression, which resulted in the muscovite dehydration melting and the formation of Miocene leucogranites. 相似文献
17.
深熔作用是大陆地壳分异、元素迁移富集和混合岩化作用的主要机制和关键地质过程.吉南地区出露的太古宙基底普遍经历了角闪岩相-麻粒岩相变质及深熔作用,长英质淡色体及淡色花岗岩广泛分布.吉南和龙花岗-绿岩地体出露的太古宙变质石英闪长岩及相关的长英质浅色体和含斜方辉石(角闪石)淡色伟晶花岗岩的野外地质特征、相互关系及岩相学特征指... 相似文献
18.
高喜马拉雅结晶岩系由中-高级变质岩和淡色花岗岩组成,是研究喜马拉雅造山带形成与演化的天然实验室.高喜马拉雅结晶岩系混合岩和淡色花岗岩中锆石和独居石的定年结果往往是分散的,对这些定年结果的解释还存在争议,严重制约了对高喜马拉雅结晶岩系变质、部分熔融作用的起始时间和持续过程的理解.对造山带中段亚东地区高喜马拉雅结晶岩系上部构造层位的乃堆拉混合岩进行了锆石U-Pb年代学研究.研究结果显示,乃堆拉混合岩暗色体给出了29.1~24.7 Ma的进变质和部分熔融的时间,混合岩浅色体获得了25.0~13.7 Ma的退变质和熔体结晶的时间,表明亚东地区高喜马拉雅结晶岩系的部分熔融作用大约开始于30 Ma并持续到13 Ma,暗示它是一个长期、持续的过程.亚东地区高喜马拉雅结晶岩系发生部分熔融的时间明显早于藏南拆离系和主中央断裂开始活动的时间,部分熔融可能在高喜马拉雅结晶岩系俯冲过程中就已经发生了.相关成果为建立造山带构造演化模型提供了新信息. 相似文献